German Patent Publication DE 298 13 080 U1 describes an arrangement having a converter, a protective device and a permanently excited motor. The protective device is provided to prevent the converter from being destroyed as a result of intermediate-circuit capacitors being reformatted and as a result of blocking voltages on the converter valves being exceeded. The protective device is connected to the motor phases between the motor and the converter. The protective device includes a means for forming a signal which signals the presence of a harmful voltage reaction and a means for shorting a terminal voltage on the motor, with a forward resistance of the shorting means acting as a braking load.
The protective device may have the following application advantage, which is not mentioned therein: when the phases in an electrical synchronous machine (for example, a permanently excited electrical machine) are shorted together, the torque produced in the motor is high only at very low motor speeds. Above these very low motor speeds, the torque produced in the motor is substantially lower in event of a short than when the motor is operated with a nonreactive load.
Technical reasons will now first of all be provided for this behaviour by synchronous machines in order to facilitate comprehension of the implementability and principal of action of the present invention. The single nonreactive resistance which is still in the circuit when the phases are shorted is the nonreactive winding resistance. For the synchronous machine as a voltage source, the synchronous reactance of the machine is the internal resistance of this voltage source. Since the synchronous reactance of the electrical machine is proportional to the speed (See, K. Wippich, Electrical Machines and Drives II, Lecture at Oldenburg University of Applied Sciences, pg. 150), the voltage drop across the synchronous reactance outweighs the voltage drop across the nonreactive winding resistance to an ever greater degree as the speed increases. This mismatch between the internal resistance (the synchronous reactance) of the synchronous machine as a voltage source and the resistance of the load (the winding resistance) severely attenuates energy transfer from the drive shaft to the nonreactive resistance in the event of a short. In addition, the energy transfer between drive shaft and nonreactive resistance in the event of a short is diminished by virtue of the armature reaction being relatively large in the event of a short (Wippich loc. cit.).